It is well known that in long distance optical communication systems degradation of the signal results from a range of different factors and variations of such factors result in signal fading. This results in reduction of signal to noise ratio and, in digital systems, an increase in the average bit error rate of detection at a receiving station. This phenomena and some methods of reduction of such problems is detailed in U.S. Pat. No. 5,361,270 the whole content of which is included herein by reference.
The invention disclosed in U.S. Pat. No. 5,361,270 employs two concatenated lithium niobate phase modulators with a 45.degree. phase shifting coupling therebetween, each phase modulator being modulated by a different wavelength signal. There is a problem with the concatenation of two phase modulators that requires two different drive frequencies to overcome modulator overlap. Such phase modulators are extremely expensive and the present invention seeks to simplify the design of a scrambler thereby to significantly reduce costs of manufacture.